/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.heron.scheduler.yarn;

import org.apache.heron.scheduler.SchedulerMain;

/**
 * {@link HeronMasterDriverProvider} is used by {@link YarnScheduler} to discover and use
 * instance of {@link HeronMasterDriver} injected by REEF framework
 */
public final class HeronMasterDriverProvider {

  /**
   * Heron-REEF scheduler is initialized by {@link SchedulerMain}. This instance of
   * {@link HeronMasterDriver} is needed by the scheduler to manage the containers.
   */
  private static HeronMasterDriver instance;

  /**
   * This is a utility class and should not be instantiated.
   */
  private HeronMasterDriverProvider() {
  }

  static synchronized HeronMasterDriver getInstance() {
    if (instance == null) {
      throw new IllegalStateException("Heron Master Driver is not initialized yet");
    }
    return instance;
  }

  static synchronized void setInstance(HeronMasterDriver instance) {
    HeronMasterDriverProvider.instance = instance;
  }
}
